History in Stone: Basic Geology of the Colorado Plateau

Anyone who has visited the incredible formations of the American Southwest immediately appreciates the power of water when given millions of years to carve up the landscape. Starting in the deepest trenches of the Grand Canyon and extending northward and skyward all the way up to the elevated hoodoos of Bryce Canyon, the mighty Colorado Plateau is a tour de force in the dynamic principles of erosion over the course of eons. Along the edges of this five-stepped behemoth are exposed layers of stone that are shaped by wind and water. Layer after layer of sand, salt and the remains of sea creatures were deposited in the ancient past by vast inland seas. Limestone and sandstone were formed and then later exposed by water and wind around the region known as the Four Corners. The various materials deposited in the area erode at different speeds and thus leave behind otherworldly structures that seem anything but natural. The unique sights that await visitors dazzle our eyes and boggle our minds as we try to decipher what on earth could have ever created such unusual places.

First called a Grand Staircase by Clarence Dutton in the late 19th century, the southwestern end of the plateau resembles five giant steps that showcase the various layers of sediment left over by prior bodies of water. Each major step is aptly named after the dominant colors displayed: the Chocolate, Vermillion, White, Gray and Pink Cliffs. Rivers, landslides, tectonic lift, fault lines, windstorms, earthquakes and countless other occurrences give each section unique formations and features. The Aquarius Plateau sits atop the Pink Cliffs and is the highest tectonic uplift in North America: Its landmark peak, Boulder Mountain, tops out at 11,317 feet. Often seen as a place of great geographic cataclysm because of the dramatic effects on the landscape, the area is considered very stable relatively speaking. Overall there has not been a lot of geologic activity to fold and fault the local stone over the last 600 million years, which allowed all these processes to slowly take place without much a lot of interruption.

Many of the areas we now consider deserts in the southwest were once covered by salty inland seas. One example that helped shape the area surrounding Moab was known as the Paradox Basin, which existed during the the Paleozoic Era (541-252 million years ago). The San Rafael Swell and Goblin Valley are excellent examples of the unusual features that occur at the shorelines of these prior oceans and lakes. The area was essentially a large sinkhole, which allowed incredible amounts of salt, sand, organic material and other chemicals to build up within. Recurring ice ages and the resulting rising and falling sea levels repeated the cycle over and over. Each layer has a different chemical recipe that accounts for different colors and traits as seen in the stratum. Iron oxide, calcium carbonate, and silica show up in various amounts and act as a natural cement for the salts and fossils. As the lower layers were covered by more and more weight above them, pressure and heat turned them from unconsolidated materials into what we call sedimentary rock.

Stratigraphy is the science of studying layers of sedimentary rock to identify the type and its approximate age. Nowhere else can you more easily see the classic teachings of Johannes Walther and his seminal Law of Facies in a real-life setting than the Colorado Plateau. While we are talking about eras that last millions of years at a time, don't think these processes only happen slowly. Keep in mind that, since 1977, a whopping 43 arches have collapsed just in Arches National Park's boundaries alone. The 130,000 square miles that the plateau covers is often referred to as Red Rock or Canyon Country, but technically it extends into areas not often considered red rock because the strata has not been exposed yet. The area is often credited with greatly advancing the science of geology in the U.S. because very few other places allowed so many visible layers to be studied so easily and from such a wide array of time periods.

There are many, many layers of identified rock to be found in the strata of the plateau, but a few of them seem to be featured more prominently and beautifully than others. The Navajo Sandstone, which blankets the area like a white turtle shell, often intermingles with the red or sometimes purple tones of the older Kayenta Formation. Below them the older Moenave and Wingate Formations form the four layers of the widely popular Glen Canyon Group. The San Rafael Group is also a famous formation because of the beautiful Entrada Sandstone layer that you can find at Cathedral Valley near Capitol Reef. The Cutler Group makes up the world famous area, Monument Valley, which makes it possibly the most widely seen formation thanks to the old Western Movies of the 1950s. At the other end of the spectrum lie the metamorphic and igneous rocks at the bottom of the Grand Canyon that are called the Vishnu Basement Group that were formed nearly 2 billion years ago.

One of the fun aspects of this area are the new and strange terms to learn while exploring canyon country. While not an exhaustive list, here are some of the key terms to know as you further your exploration of geology in the Southwest.

• Names that reference water in these formations are an homage to liquid design: Multi-step cliffs are "reefs," vertical lips are "fins," and the "waterpocket fold" created Capitol Reef and even the famous sandstone formation known as "The Wave."
• A hoodoo is the general term for spires, vertical shafts, balancing rocks, and any other column-like formation created by the erosion of softer rock around a harder deposit.
• Natural arches, such as Corona Arch, are formed by wind and sand, while natural bridges are made by water tunneling under sandstone. These tend to be larger, and Owachomo Bridge and Rainbow Bridge are stunning examples.
• Domes are popular in Capitol Reef (hence the name Capitol Dome), and these are often made of the white rippled sandstone of the newer Navajo Formation.
• A Moqui Marble is a small, hard ball made of sandstone and iron oxide.
• Slickrock, which is not very slick at all usually, is beloved by mountain bikers for its windswept polish that gives it such a smooth ride.
• Slot canyons are small pinched areas that hopefully are only large enough to allow a person with a backpack to slide through for dramatic effect, but they do pose a high risk for flash floods.

We all know how great nature is for our body and our spirit, but the Colorado Plateau is also a great place to stimulate our curiosity and sense of wonder. The Paiute people once called Bryce Canyon "Angka-ku-wass-a-wits," which means "people with red painted faces" and refers to the red rock hoodoos that stand up like humans frozen in time. Spoken tradition tells of the time when the powerful spirit Coyote turned the Legend people into stone for the bad deeds they had committed. While exploring the secluded backcountry of the plateau, it is hard to deny there is a palpable magic in the air. Perhaps science doesn't tell the entire story in these parts after all.